In the production of a semiconductor device, a transfer pattern is formed by, for example, irradiation of exposure light through a photomask (reticle).
As such a photomask, a photomask comprising a transparent substrate and a light-shielding film pattern formed thereon has been conventionally used. As a material of the light-shielding film, a chromium-based material (chromium alone, a material containing chromium and nitrogen, oxygen, carbon, or the like, or a layered film composed of these material films) has been generally used.
Further, in recent years, phase shift masks have been put to practical use in order to improve the resolution of transfer patterns. Various types (Levenson type, auxiliary pattern type, self-aligned type, etc.) of phase shift masks are known. Among them, a halftone phase shift mask suitable for high-resolution pattern transfer of holes and dots is known.
In the halftone phase shift mask, a light-semitransmissive film pattern having a phase shift of about 1800 is formed on a transparent substrate and the light-semitransmissive film is formed by a single layer or multilayers. For example, the publication of Japanese Patent No. 2966369 discloses a light-semitransmissive film pattern which is formed by a thin film made of a substance containing metal such as molybdenum, silicon, and nitrogen as main components.
The light-semitransmissive film made of such a material can control a predetermined phase shift amount and transmittance by a single layer and further is excellent in acid resistance, light resistance, and so on.
As described above, as the film materials used in the photomasks, there have been developed not a few materials containing nitrogen for various reasons.
On the other hand, when pattern transfer is performed using a photomask (reticle), high-energy laser light is irradiated onto the photomask. As a result, chemical reactions on the photomask surface are accelerated by the laser irradiation. Therefore, formation of some deposits are accelerated so that the deposits are produced and adhered on the photomask as foreign substances. Ammonium sulfate has been confirmed as one of such deposits.
The photomask is genarally cleaned by the use of a sulfuric acid-based cleanser in the final process. It is considered that sulfuric acid or sulfuric acid ions derived from the sulfuric acid-based cleanser used in the cleaning process often remain on the photomask after the cleaning. Therefore, it is considered that reactions between the sulfuric acid ions and ammonium ions produced for some reason are accelerated by the laser irradiation so as to cause the production of the deposits.
Particularly, following finer formation of LSI patterns in recent years, the wavelength of exposure light sources (exposure light wavelength) has been changing toward a shorter wavelength, i.e. from a current KrF excimer laser (248 nm) to an ArF excimer laser (193 nm). Under these circumstances, in case where, a short-wavelength exposure light source such as the ArF excimer laser is used, the laser output becomes further higher. As a result, the formation of the deposits problematically tends to be readily accelerated so that the production of the foreign substances becomes more outstanding.
It is considered that a generation source of the ammonium ions is a substance or an adhering substance existing in the atmosphere or derived from a pellicle. However, as a result of researching thin films using materials containing nitrogen which are used in photomasks as described above, it has been found that more ammonium ions (NH4+) exist on the surfaces of the thin films containing nitrogen than on the surfaces of thin films containing no nitrogen. Therefore, it is considered that the nitrogen-containing thin film may contribute to the deposition of ammonium sulfate that can be a particle defect.
The present invention has been made in terms of the foregoing prior art problems and has an object to provide a lithography mask which is capable of reducing the production of ammonium ions caused by a component of a thin film, and a lithography mask blank which is capable of producing such a lithography mask.
Another object of the present invention is to provide a halftone phase shift mask which is capable of reducing the production of ammonium ions caused by a component of a thin film, and a halftone phase shift mask blank which is capable of producing such a halftone phase shift mask.